I'm fairly sure we don't have enough information here.
I also don't believe the desktop version of iTunes plays movies in 4K.
Also,... you can't download 4K content from iTunes. 4K is streaming only.
You may be interested in the ATV4K ... it is the ONLY way to watch iTunes 4K content that I know of so far and it's very easy to use.
It's my favorite thing ever. And it's not expensive.